There is a wide consensus that an HIV vaccine should elicit protective antibodies. To ensure that optimal responses are elicited, it is important to understand how antibodies exert their protective activity in vivo. For instance, is it only necessary to elicit antibodies that can neutralize in vitro, as is the current focus of immunogen development? Or are other antibody functions, present in vivo but not present in in vitro neutralization assays, important? If so, which functions? We are approaching the mechanism(s) of antibody protection in vivo using the SHIV/macaque model and a broadly neutralizing human anti-HIV antibody (b12) that we have studied intensively in the laboratory. In particular, we are attempting to probe mechanism by investigation of the protective effects of engineered variants of b12 whose ability to interact with other compartments of the immune system has been altered. We have provided evidence to suggest that the interaction of antibody and host cell Fc receptors is important in protection and now propose to explore this involvement more deeply to determine the features of antibody responses, in addition to neutralization, that will provide benefit in the context of a vaccine. The specific aims are: (1) To determine the roles of the key Fc receptors FcRIIa and FcRIIIa in antibody protection against mucosal HIV challenge (2) To determine the effects of greatly enhancing the major FcR functions, antibody-dependent cellular cytotoxicity (ADCC) and phagocytosis, on antibody protection against mucosal HIV challenge and (3) To investigate in vitro how monoclonal and polyclonal anti-HIV antibodies mediate the extra-neutralizing activities shown in Aims 1 and 2 to be crucial in antibody protection. These studies will help define the antibody specificities, and particularly combinations of specificities, most likely to offer optimal protection against HIV exposure.